1. Field of the Invention
This invention relates to a packaged semiconductor device. In particular, it concerns the improvement of a packaged semiconductor device having a heat sink insulated from the semiconductor chip.
2. Description of the Prior Art
A known configuration for power semiconductor devices with e.g. a power transistor array is that in which a plurality of semiconductor chips are mounted on one heat sink and sealed with a single layer of molded resin. In this type of semiconductor device, the individual semiconductor chips and the heat sink must be insulated from each other. For this reason, a ceramic substrate is used initially, but owing to the high cost of this substrate and problems with its workability, various substitutes are proposed.
A construction for a packaged semiconductor device which did not employ a ceramic substrate is proposed. The semiconductor chips could be insulated from each other, using an ordinary metal heat sink. This construction is explained below, referring to FIGS. 1 through 4.
FIG. 1 is a plan view of a heat sink 1 consisting of a metal plate of high thermal conductivity, of aluminium or the like. Screw holes 11 are formed in this heat sink 1, for securing to a cooling body. FIG. 2 is a plan view of a lead frame 2 patterned with thin plate of conductive metal such as copper or steel alloy. As the drawing shows, four independent bed parts 21 and lead patterns 22 are formed on lead frame 2, and these are supported by being connected to a frame 23.
To obtain this construction, the semiconductor chips are die-bonded on to bed parts 21 of lead frame 2, and the necessary wire-bonding is then effected. Then the frame is placed in a metal mold die set together with heat sink 1 and transfer molding is carried out. When this is done, lead frame 2 is placed over heat sink 1 with a prescribed gap. This gap is then filled with molded resin. FIG. 3 is a plan view showing the state after the sealing with resin. Numeral 3 in the drawing is a molded layer.
FIG. 4 is a sectional view along the line V--V in FIG. 3. As shown in FIG. 4, a semiconductor chip 4 is die-bonded on to bed part 21 via a solder layer 5. Bonding wires 6 are interconnected between chip 4 and lead frame 2. As the drawing shows, in this construction molded resin layer 3 is filled in the gap between lead frame 2 (in particular bed part 21) and heat sink 1, and by this means insulation is achieved between the two.
Thermal dissipation characteristics in the construction described above depends on the thermal conductivity of the molded resin layer interposed between bed part 21 and heat sink 1. The resin used for molded resin layer 3 is therefore epoxy resin with its thermal conductivity Tc increased by an admixture of crystalline silica powder (Tc=2.5 W/(m.multidot.K)).
The admixture of a large quantity of crystalline silica unavoidably increases the viscosity of the resin of high thermal conductivity which used in existing packaged semiconductor devices. For this reason, voids are liable to occur, when forming resin layer 3 by transfer molding, in the resin layer which fills the space between bed part 21 and heat sink 1. These voids reduce the breakdown voltage. This tendency is proportional to the viscosity of the melting resin, and in inverse proportion to the distance between the bed part and the heat sink. The problem arose that when an attempt is made to increase the thermal dissipation characteristics by increasing the silica content, the marked generation of voids resulted in reduced breakdown voltage. Other problems caused by increased viscosity of the melting resin are greater flow resistance of the melting resin and poor bonding, with breaking of bonding wires, etc.
When an attempt is to be made to increase the silica content and yet at the same time to inhibit the generation of voids, the distance between the bed part and the heat sink must be made greater. In that case the thermal resistance of the resin layer is increased and improvement in thermal dissipation characteristics is no longer obtained. It may be noted here that when epoxy resin, containing crystalline silica, of Tc=2.5 W/(m k) is used for the molding, with a bed 21 of 8 mm.times.8 mm, the generation of voids made it impossible to reduce the distance between the bed part and the heat sink below 0.5 mm.